Re: Map Block number from hdd to md

[Keld Simonsen <keld@keldix.com>]

On Thu, Feb 18, 2010 at 10:47:55AM +1100, Neil Brown wrote:
> On Tue, 16 Feb 2010 12:14:38 +0100
> Michael <michael@rw23.de> wrote:
> 
> > On Tue, 16 Feb 2010 12:20:14 +1100, Neil Brown <neilb@suse.de> wrote:
> > >> is there any method to find that bad block in context of the raid block
> > >> device? reading all files is not a good option on large raidsets.
> > >> level 5, 64k chunk, algorithm 2
> > > 
> > > It isn't that hard.  The code is in drivers/md/raid5.c in the
> > kernel.....
> > > 
> > > Rather than trying to describe in general, give me the block number,
> > > device,
> > > and "mdadm --examine" of that device, and I'll tell you how I get the
> > > answer.
> > 
> > 
> > the bad block number was 122060740 sec.
> > 
> > [root@raw sqla]mdadm --examine /dev/sda3
> > /dev/sda3:
> >           Magic : a92b4efc
> >         Version : 0.91.00
> >            UUID : 9815a2c6:c83a9a53:2a8015ce:9d8e5e8c (local to host raw)
> >   Creation Time : Thu Feb 11 16:01:12 2010
> >      Raid Level : raid6
> >   Used Dev Size : 966060672 (921.31 GiB 989.25 GB)
> >      Array Size : 2898182016 (2763.92 GiB 2967.74 GB)
> >    Raid Devices : 5
> >   Total Devices : 5
> > Preferred Minor : 2
> > 
> >   Reshape pos'n : 974014464 (928.89 GiB 997.39 GB)
> >      New Layout : left-symmetric
> > 
> >     Update Time : Tue Feb 16 11:58:37 2010
> >           State : clean
> >  Active Devices : 5
> > Working Devices : 5
> >  Failed Devices : 0
> >   Spare Devices : 0
> >        Checksum : 16372b12 - correct
> >          Events : 363519
> > 
> >          Layout : left-symmetric-6
> >      Chunk Size : 64K
> 
> So...
> There is no Data Offset give, so it is zero.  so the block is 122060740
> sectors into the data area of the devices.
> Chunksize if 64k (128 sectors), so 
> 122060740 / 128 == 953599 remainder 68.
> So Stripe number 953599, and sector 68 of device '2' of that stripe.
> 
> A stripe had 4 disks when raid5, 5 when raid6, so 3 data drives.
> So stripe 953599 is 953599 * 3 * 128 sectors from the start of the
> array. i.e. 366182016 sectors.
> 
> In the raid5 layout:
> 4 drives, so 4 different stripe layouts.
> 953599 % 4 == 3, so it is layout 3 (of 0, 1, 2, 3).
> Looking at the code in raid5.c for LEFT_SYMMETRIC 
> The parity disk is disk 0. The data disks follow that,
> so device '2' holds data chunk '1'.
> So we add 1 full chunk plus the 68 sectors of the partial chunk.
> i.e. that sector is 366182016 + 128 + 68
>  or sector 366182212 in the array.
> 
> After the conversion to RAID6, there are 5 drives so 5 stripe layouts.
> 953599 % 5 == 4, so layout 4
> So 'P' is device 0, 'Q' is device 1, D0 is device 2 etc.
> So sda3 is the first data disk in the stripe, so there are no full stripes to
> add, just the partial stripe.
> 366182016 + 68 == 366182084

Sounds like it would be nice to have a program to calculate this :-)

Seriously, Neil, would it be possible to lift out the code from the
kernel to make a small utility - for salvaging raid file systems?

best regards
keld